Such radiation-emitting semiconductor diodes, especially if the wavelength of the emission is in the visible part of the spectrum, are suitable radiation sources--if designed as diode lasers--for inter alia information-processing systems such as laser printers with which information is written, optical disc systems such as Compact Disc (Video) (CD(V)) players or bar code readers, by which information is read, and Digital Optical Recording (DOR) systems, by which information is written and read. There are also numerous applications in optoelectronic systems for LED versions of such diodes.
Such a radiation-emitting diode and such a method for manufacturing same are known from the article "AlGaInPP Double Heterostructure Visible-Light Laser Diodes with AlGaInPP Active Layer Grown by Metalorganic Vapor Phase Epitaxy" by K. Kobayashi et al., published in IEEE Journal of Quantum Electronics, vol. QE-23, no. 6, June 1987, p. 704. In this article, a radiation-emitting semiconductor diode is described on which an active layer of InGaP is present on a substrate of n-GaAs between two cladding layers of InAlGaP. The semiconductor materials of these layers each comprise a mixed crystal having two sublattices in which the phosphorus atoms are present on the one sublattice and the atoms of the other elements, in this case In and Ga atoms for the active layer and In, Al, and Ga atoms for the cladding layers, are present on the other sublattice. A buffer layer of GaAs is present between the substrate and the first cladding layer. The wavelength of the emission of the diode, which is constructed as a laser here, is approximately 670 nm (i.e. the wavelength in photoluminescence is approximately 660 nm, which corresponds to a band gap of approximately 1.88 eV).
A disadvantage of the known radiation-emitting semiconductor diode is that the experimentally found wavelength for the emission is higher than the theoretically expected one: for example, the wavelength expected for an InGaP active layer is approximately 650 nm, whereas approximately 670 nm or more is often found in practice. A similar effect occurs in the case of cladding layers comprising InGaAlP, where the experimentally found band gap again is less than the theoretically expected one. The band gap of both the active layer and the cladding layer may be increased by increasing the aluminium content of these layers. This possibility is limited, especially for the cladding layers which contain indirect semiconductor materials, because progressive addition of aluminium results in an ever smaller increase in the band gap, and doping of the cladding layers becomes more difficult. As regards the active layer, another possibility is to make the latter thinner, which, however, renders manufacture more difficult. It was found experimentally that the use of misoriented substrates, for example (311) or (511) substrates, causes the experimental band gap--and thus the wavelength of the emission--to lie (much) closer to the theoretically expected value. The use of misoriented substrates, however, is more expensive and has the drawback that it restricts the choice of the longitudinal direction of the resonance cavity.
The present invention has for its object inter alia to provide a radiation-emitting semiconductor diode--especially a semiconductor diode laser--which does not have the previous disadvantage, or at least to a much lesser extent, and therefore combines a lowest possible emission wavelength with a highest possible band gap of the cladding layers. The present invention has for its further object to realise a diode having an active layer which comprises InGaP with a band gap which is equal to approximately 1.94 eV, which corresponds to a wavelength of approximately 650 nm for a diode laser (the wavelength in photoluminescence is then approximately 640 nm). In particular, the invention has for its object to realise such a diode which emits at a wavelength of 633 nm, which is exactly the wavelength of a helium-neon gas laser. The invention also has for its object to provide a simple method of manufacturing such a radiation-emitting semiconductor diode.
According to the invention, a radiation-emitting semiconductor diode of the kind described in the opening paragraph is for this purpose characterized in that the buffer layer comprises aluminium-gallium arsenide (AlGaAs), of which the aluminium content has at least a minimum value belonging to the band gap of the active layer. Failure of the band gap to attain the theoretically expected value is found to be caused by an ordering occurring in the crystal (sub)lattice of the III-elements in the mixed crystals of the cladding layers and the active layer. It appears to be possible for such an ordering to occur in mixed crystals of various materials, the ordering taking place for the materials used here in the 178 (111)A planes and resembling the ordering of the so-called CuPt structure. Besides the substrate orientation, growing conditions such as the growing temperature were found to have an influence on whether or not this ordering takes place. Especially at a comparatively high growing temperature, little or no ordering takes place so that the band gap of the layer provided a maximum. In comparison, when the known buffer layer is used, however, the crystal structure of InGaP or InAlGap semiconductor layers manufactured at higher temperatures is poor, which is very detrimental to the quality of the radiation-emitting semiconductor diode. A buffer layer comprising AlGaAs with an aluminium content higher than or equal to a minimum value belonging to the band gap of the active layer is suprisingly found to render it possible to apply the InGaP or InAlGaP layers at higher than the usual temperatures--and consequently with a more disorderly distribution of the atoms of the remaining elements over the other sublattice--while nevertheless semiconductor layers having a very good morphology and crystal structure are obtained. The semiconductor layers obtained as a result have a band gap which corresponds to a more random distribution of the atoms of the remaining elements over the other sublattice. This means for the semiconductor materials used here that, for example, In.sub.0.49 Ga.sub.0.51 P has a band gap of approximately 1.85 eV (with a highly ordered structure) and a band gap of approximately 1.94 at a substantially completely disordered distribution, which corresponds to an emission wavelength of approximately 680-650 nm. In the case of In.sub.0.5 Al.sub.1.10 Ga.sub.0.15 P, for example, the band gap (in the case of substantially complete disorder) is approximately 2.05 eV and for In.sub.0.5 Al.sub.0.30 Ga.sub.0.20 P this is 2.3 eV, which are higher values than those found for the same materials in an orderly structure. For an InGaP active layer having a band gap of approximately 1.88 eV, for example, the minimum aluminium content of the buffer layer is approximately 6 atom percent. For an InGaP active layer having a band gap of approximately 1.92 eV, the minimum aluminium content of the buffer layer is approximately 9 atom percent. No upper limit for the aluminium content of the AlGaAs buffer layer was found. Preferably, however, the aluminium content is chosen to be smaller than 100 atom percent since pure AlAs is rather hygroscopic, which may lead to problems when the cleavage surfaces to be formed are exposed to the air. To safeguard a good electric conduction, the aluminium content may be further reduced to those contents at which a relatively low resistivity is still possible. No lower limit was found for the thickness of the buffer layer: buffer layers having a thickness of 6 .ANG. and an aluminium content of 20% were also very satisfactory. Good results were obtained with a buffer layer thickness between 0.1 and 1 .mu.m. It should be noted that there is an AlGaAs layer between a DH (Double Hetero) structure and the substrate in the device described in the article "Fabrication and optical characterization of first order DFB InGaP/AlGaInP laser structures at 639 nm" published in Electronics Letters, 26th Apr. 1990, vol. 26, no. 9, p. 614. This does not relate to a radiation-emitting diode, however, since no pn junction is present, neither is an aluminium content specified or suggested. In addition, it is apparent from the specified wavelength of the emission of the InGaP, namely a photoluminescence wavelength of 670 nm (i.e. the band gap corresponds to 1.85 eV) that the InGaP has a very strongly ordered distribution of the atoms of the remaining elements over the other sublattice (the wavelength of the emission will be approximately 680 nm in a version designed as a diode laser). In the radiation-emitting semiconductor diode according to the invention, the substrate used may be a (substantially perfectly oriented) (001) substrate, which is an important advantage, as explained above. Preferably, a substrate of gallium arsenide is used, but other substrates, for example a silicon substrate, may alternatively be used. In a favourable embodiment, the substrate comprises gallium arsenide which is provided with a conducting layer at the lower side, an intermediate layer of indium-gallium phosphide or aluminium-gallium arsenide and a contact layer of gallium arsenide, which layers are of the second conductivity type, are present in that order on the upper cladding layer, and the semiconductor body comprises a mesa-shaped strip adjoining its surface, which strip comprises at least the contact layer and is coated with another conducting layer which extends to outside the mesa-shaped strip and beyond. This strip forms a junction constituting a barrier with a layer situated below it. In this embodiment, the current is very effectively limited to the active region situated below the mesa-shaped strip, which promotes a low starting current. This embodiment has the additional advantage that the radiation-emitting semiconductor diode is of a simple structure and very easy to manufacture: the other conducting layer can be provided without a masking step. To realise a wavelength which is as low as possible, an active layer is preferably used which comprises a multiple quantum well structure with well layers of InGaP and barrier layers of InAlGaP. No very thin layers are required in a radiation-emitting semiconductor diode according to the invention for obtaining a substantial increase of the wavelength: quantum well and barrier layers having a thickness between approximately 4 and 6 nm are already sufficiently thin and are comparatively easy to manufacture. Thus radiation-emitting semiconductor diodes according to the invention were manufactured having an emission wavelength of 633 nm, which is equal to the wavelength of an He-Ne gas laser, where the active layer was formed with eight InGaP quantum well layers having a thickness of approximately 5 nm separated by barrier layers having a thickness of approximately 4 nm. The cladding layers formed a so-called SCH (Separate Confinement Hetero) structure with aluminium contents of 25 at % for the separate confinement layers and 35 at % for the cladding layers.
A method of manufacturing a radiation-emitting semiconductor diode according to the invention is characterized in that the semiconductor material chosen for the buffer layer is aluminium-gallium arsenide with an aluminium content which is at least equal to a minimum value belonging to the band gap of the active layer, while a growing temperature is chosen higher than 700.degree. C. In this way the disorder in the distribution of the atoms of the elements other than phosphorus over the sublattice in which these elements are present is promoted, so that diodes having the desired characteristics are obtained. Owing to the comparatively high temperature, semiconductor layer structures with excellent characteristics, such as a good morphology, a good crystal structure, and good interfaces, are obtained. Preferably, a growing temperature of at least 730.degree. C. is chosen, while the best results are obtained with a temperature of approximately 760.degree. C. with the use of MOVBE as the growing technique. A minimum aluminium content value for the buffer layer of approximately 6 at % belongs to a growing temperature of approximately 730.degree. C. At 760.degree. C. the minimum aluminium content is approximately 9 at %. In this process, the substrate chosen is preferably a (001) substrate, and the ratio of the quantity of III and V elements offered during the application of the semiconductor layers is chosen to lie between approximately 100 and 400. These V/III ratios were found to be favourable for obtaining the greatest possible disorder in the mixed crystals of the semiconductor layers formed.